A fraud detector including detection means for detecting the presence of an un-authorized electric wire inserted in the channel and which is in contact with said contact elements, is already known in the art, e.g. from the published French patent application 2.554.262. Therein the detection means include an electrically conductive plate mounted near to or in the channel so as to detect un-authorized or fraudulent electric wires inserted in this channel by means of the capacitive effect produced between the wire(s) and the plate.
A drawback of this known fraud detector is that the thickness and the dielectrical properties of the electric insulation as well as the diameter of the conductor forming the fraudulent wire may be such that the value of the capacitance created between this wire and the plate is close to that of a parasitic capacitance so that the detection means and thus also the fraud detector work improperly. Also the position of the wire in the channel may be such that it cannot be detected correctly by these detection means, e.g. when the wire is located too far from the plate.